1. Field of the Invention
The present invention concerns a variable pitch blade propeller hub for a turbomachine of the unducted fan (“open rotor”) type.
2. Description of the Related Art
The fan of a turbomachine of this type typically comprises two contrarotating coaxial external propellers, respectively upstream and downstream, each of which is driven in rotation by a turbine of the turbomachine and extends substantially radially outside the nacelle of the turbomachine.
Each propeller usually comprises a hub of the type including a polygonal ring to support said blades, concentric with the longitudinal axis of the turbomachine and composed of two parallel annular end flanges between which are diametrically fastened, by root areas arising from the annular flanges, collars with cylindrical radial housings to receive said blades. The collars are equi-angularly distributed on the lateral periphery of the polygonal ring and separated from each other by plane intermediate areas with through-openings or orifices, and connecting means connect the polygonal ring to a turbine rotor element of the turbomachine.
The blades can turn in the housings of the collars of the polygonal ring and to this end are driven in rotation about the axes of the blades by appropriate means to adjust the angular pitch of the blades and to optimize it as a function of the operating conditions of the turbomachine.
In operation, the rotating parts of the turbomachine, and notably the hub and the blades of the propeller, are subjected, to various degrees, to high loads of mechanical, thermal, aerodynamic, etc. type. In particular, the cylindrical housings of the blades tend to be deformed by the high forces that are exerted at the level of the radial collars of the polygonal ring, until they assume an oval configuration. As the root areas connecting the collars to the annular flanges are wide and extend over a large peripheral portion of the collars for reasons of mechanical strength and stiffness, the traction forces exerted by these flanges on the collars cause them to assume an oval shape. Thus there is a risk that the raceways of the bearings that are provided between the cylindrical housing of each collar and a rotary device (notably a plate and crown device) carrying the blade and enabling the pitch thereof to be varied, may deteriorate rapidly, to the point of no longer guaranteeing correct functioning of the blades, with the consequences to which this may lead.
Moreover, to reduce the mass of the polygonal ring, which may have a diameter greater than one meter, the intermediate areas of the lateral periphery of the ring are provided with through-openings formed alternatively and consecutively between the collars with radial housings receiving the blades. As a result, there is a significant saving in mass without this compromising the mechanical characteristics of the polygonal ring.
Moreover, the use of composite materials is constantly expanding, again to lighten the parts and also for reasons of stiffness and longevity. Also, the propeller blades are advantageously manufactured from these composite materials.
It has nevertheless been noticed that, in operation, the roots of the blades, connected to the exterior side of the polygonal ring, as opposed to its interior side facing toward the turbines, are subjected to relatively high temperatures caused notably by the flow of hot air in the turbine element cooling circuit. Some of this hot air passes through the through-openings in the intermediate areas of the polygonal ring and flow around the roots of the blades, heating them. The composite materials of the blades are incompatible with these temperatures. Moreover, the combustion gases passing coaxially through the turbomachine contribute to maintaining these high temperature levels, notably by conduction via the parts themselves, despite the presence of the cooling circuits.